Cavity damage accumulation and fracture in SiO2-doped zirconia during superplastic deformation

The tensile flow stress of a yttria-stabilized zirconia (3Y-TZP) decreased continuously with an increase in SiO2 addition up to 2.5 mass%. The lowered flow stress was, however, accompanied by no enhanced tensile ductility except for either SiO2 doping less than 0.1 mass% or that of 2.5 mass%. Stereological microanalysis of deformed specimens revealed that the ductility was controlled strongly by the cavity morphology which depended on the amount of SiO2 phase. In materials doped with 0.0–0.1 and 2.5 mass% SiO2, round cavities grew into sizes smaller than 100 μm. The analysis indicated that the cavity growth law was the same in these materials and thus the ductility was controlled mainly by the cavity nucleation rate. On the other hand, the intermediate SiO2 doping superinduced the crack-like cavities with growth rates about 3 times higher than that of the round ones. In spite of their low population, they developed easily into cracks with sizes of hundreds of micrometers, aligning perpendicular to the stress axis and resulting in a limited tensile ductility. The cause of the crack-like cavitation was also discussed on the basis of transmission electron microscopy (TEM) observations.